The term liquid crystal or mesogen refers to states of matter which lies between solid crystals and isotropic liquids. Liquid crystalline materials possess some structural characteristics of crystals yet may be viscous or mobile liquids.
The varying degrees of order which are possessed by liquid crystals can be classified into two general types of structures called mesophases. A liquid crystal, when heated from the crystalline state may initially lose one dimension of its positional order to form a smectic mesophase. The smectic phase retains the orientational order of the crystalline state.
Further heating may result in the formation of a nematic mesophase. In this phase, the positional order is lost and the material retains only the one-directional crystalline state. These mesophases can be exhibited by both low molecular weight compounds and by polymers with mesogenic groups.
There are two main classes of thermotropic polymers. The mesogenic groups can incorporated into the backbone of the linear polymer or attached as side-groups. For main-chain thermotropic polymers, such as those described in the present invention, the polymeric and mesogenic properties are closely coupled since the polymer molecule adopts a conformation and packing which is compatible with the structure of the particular mesophase. In this case alteration of the repeating unit will affect the molecular packing and cause the properties of the mesophase to depart from those of the low-molecular weight mesogen. The mesogen may also strongly affect the mechanical properties of the solid polymer cooled from the mesophase.
Generally speaking, the development of a melt processible polymer liquid crystalline coating system requires materials with low melting points. Several approaches have been used to lower the melting points of thermotropic polyesters to enable them to be melt processed without decomposition. These methods include the use of flexible spacers, lateral substituents within the mesogen, bent and crankshaft monomers and copolymerization.
Most of these liquid crystalline polyesters can only be effectively produced by melt polymerization owing to their low solubility characteristics. Nevertheless, melting points ranging from 100.degree. to 300.degree. C. have been achieved with these approaches. For example, R. W. Lenz, J. Polym. Sci., Polymer Symposium 72, pp. 1-8, (1985), describes a series of polyesters based on a triad of 1,4-phenylene units linked by ester bonds and having an oxyethylene spacer unit. With sufficient number of oxyethylene units, a melting point as low as 102.degree. C. was obtained using a polydisperse oxyethylene spacer. These polyesters generally display broad smectic mesophases and have low solubility in common organic solvents at ambient temperatures.
A homologous series of thermotropic polyesters based on 4,4'-dihydroxy-2,2'-dimethylazoxybenzene and alkanedioic acids were reported in Blumstein, A. and Thomas, O., Macromolecules, 1982, Vol. 15, No. 5, pp. 1264-7. These polyesters were disclosed to have moderate transition temperatures and to be soluble.
In low molecular weight liquid crystals, cyclohexyl moieties have been used to replace aromatic rings in mesogens to both lower the crystal melting point and increase the breadth of the nematic phase. See G. Gray, Polymer Liquid Crystals, A. Ciferri, Ed., Academic Press, New York, 1982.
In polymers, cyclohexyl moieties also have been used in the formation of rigid main-chain polymers, block copolyesters, and semi-flexible polyesters containing mesogens and flexible spacers. See Schaefgen, U.S. Pat. No. 4,118,373; M. Polk, K. Bota, E. Akubiro, Macromolecules, Vol. 14, pp. 1626-9, (1981); D. Bruan and U. Schulke, Makromol. Chem., Vol. 187, pp. 1145-50, (1986); and Kim Clausen et al., Macromolecules, Vol. 20, pp. 2660-4, (1988).
In semi-flexible polymers, trans-1,4-cyclohexanediol and trans-1,4-cyclohexane diacid have been used in mesogens together with methylene spacers to give polyesters with improved solubility and lower crystal melting points. See Clausen et al., supra. These polyesters have melt transitions generally lower than those of the corresponding fully aromatic triad polyesters. The lowest melting point was 144.degree. C. with an isotropic transition of 205.degree. C.
It is an object of the present invention to provide polyesters with low melting points, a broad nematic mesophase and high solubility characteristics.
A further object of the present invention is to provide processing and curing conditions that allow facile alignment of the polyesters and the subsequent formation of a three-dimensional polymer network.
Another object is to provide a thermotropic, nematic polyester having a cyclohexane ring in the backbone and a crystalline melting point of less than 125.degree. C., preferably less than 100.degree. C.
An additional object is to provide a thermotropic, nematic polyester having a cyclohexane ring in the backbone and which has a solubility in halogenated solvent greater than 10 wt. %, preferably greater than 20 wt. %, and more preferably at least 30 wt. %.
A further object is to provide a thermotropic, nematic polyester having a cyclohexane ring in the backbone and which has a nematic range of at least 20.degree. C., preferably at least 40.degree. C., and more preferably at least 60.degree. C.